This invention relates to serial impact printers and, more particularly, to serial impact printers employing rotatable character arrays such as a print wheel or daisy.
A print wheel of the daisy type, as shown in U.S. Pat. No. 3,949,853--Lahr et al, comprises a central hub portion which is removably coupled to the drive mechanism for the print wheel and a plurality of petals or radially-extending spokes which carry the various character elements at the radial extremities thereof in circumferentially spaced positions about the print wheel. Print wheels or daisies of this type may be utilized in a variety of applications. For example, print wheels may be used in serial printers such as those manufactured by Qume and Diablo, which are associated with communications terminals, computer output devices and other printing applications in the data processing field. In addition, such print wheels may be and are utilized in typewriters including equipment manufactured by Xerox Corp.
One important consideration in mounting a print wheel of the daisy-type is the manner in which the print wheel may be removed from the rotary drive mechanism. Removability and/or replacement of the print wheels is particularly important since the print wheels are subjected to a substantial battering force by the hammer. In addition, it is desirable, particularly in typewriter applications, to permit removability or replacement of the print wheel so as to allow for selection of a particular style of type by merely changing print wheels.
Replacement or removability of print wheels has been achieved in the Xerox typewriters which employ daisies by hinging the print wheel carriage so as to allow the print wheel and drive to be moved upwardly and away from the opposing platen. Once moved to this position, the print wheel may be extracted by pulling the print wheel from the end of the drive shaft since the platen is no longer in a position of interference with respect to that drive shaft. See also U.S. Pat. No. 3,707,214--Ponzano. Replacement of the daisy has also been achieved by hinging the printing hammer as shown in U.S. Pat. No. 3,651,916--Bacchi, so as to allow the daisy to be removed from the drive shaft.
As indicated above, the print wheels of the prior art have been driven by means of drive shafts coupled to a suitable drive which may include a stepper motor as shown in the aforesaid Ponzano patent. Magnetic elements have been proposed for use in print wheels, as shown in U.S. Pat. No. 3,842,960--Gerry, for purposes of determining the print wheel position, but the magnetic elements are not utilized to drive the print wheel.
Another important consideration in print wheels and the associated drive is the achievement of a high performance level. In this connection, it will be apprecitaed that the characters must be moved rapidly to the impact printing position if a high serial printing rate is to be achieved. This becomes particularly important in data processing printers and typewriters which are capable of operating in a memory mode. For similar reasons, it is particularly important to minimize the mass of the print wheel drive where the drive and the print wheel are mounted on a movable carriage so as to assure rapid movement along the platen.